Replace the term "Log In" with "Sign In".

pmpro-customizations.php

<?php

/**
 * This recipe replaces the term "Log In" with "Sign In".
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_gettext_signin( $output_text, $input_text, $domain ) {
	if ( $output_text === 'Log In' ) {
		$output_text = 'Sign In';  // replace text in quotes with your custom text
	}
	return $output_text;
}
add_filter( 'gettext', 'my_gettext_signin', 10, 3 );